Reaction Mechanisms Underlying Unfunctionalized Alkyl Nitrate Hydrolysis in Aqueous Aerosols

Alkyl nitrates (ANs) are both sinks and sources of nitrogen oxide radicals (NOx = NO + NO₂) in the atmosphere. Their reactions affect both the nitrogen cycle and ozone formation and therefore air quality and climate. ANs can be emitted to the atmosphere or produced in the gas phase. In either case, they can partition into aqueous aerosols, where they might undergo hydrolysis, producing highly soluble nitrate products, and act as a permanent sink for NOx. The kinetics of AN hydrolysis partly determines the extent of AN contribution to the nitrogen cycle. However, kinetics of many ANs in various aerosols is unknown, and there are conflicting arguments about the effect of acidity and basicity on the hydrolysis process. Using computational methods, this study proposes a mechanism for the reactions of methyl, ethyl, propyl, and butyl nitrates with OH^– (hydroxyl ion; basic hydrolysis), water (neutral hydrolysis), and H₃O⁺ (hydronium ion; acidic hydrolysis). Using quantum chemical data and transition state theory, we follow the effect of pH on the contribution of the basic, neutral, and acidic hydrolysis channels, and the rate coefficients of AN hydrolysis over a wide range of pH. Our results show that basic hydrolysis (i.e., AN reaction with OH^–) is the most kinetically and thermodynamically favorable reaction among our evaluated reaction schemes. Furthermore, comparison of our kinetics results with experimental data suggests that there is an as yet unknown acidic mechanism responsible for acidic catalysis of AN hydrolysis.